Since the pioneering work of Ruff et al. (1934) Z. Anorg. Allg. Chem. 217:1, and of Rudorff et al. (1947) Z. Anorg. Allg. Chem. 253:281, graphite has been known to react with elemental fluorine at high temperatures to yield graphite fluoride compounds of general formula (CFx)n. Systematic studies on the fluorination reaction later showed that the resulting F/C ratio is largely dependent on the fluorination temperature, the partial pressure of the fluorine in the fluorinating gas, and physical characteristics of the graphite precursor, including the degree of graphitization, particle size, and specific surface area. See Kuriakos et al. (1965) J. Phys. Chem. 69:2272; Nanse et al. (1997) Carbon 35:175; Morita et al. (1980) J. Power Sources 5:111; Fujimoto (1997) Carbon 35:1061; Touhara et al. (1987) 2. Anorg. All. Chem. 544:7; Watanabe et al. (1974) Nippon Kagaku Kaishi 1033; and Kita et al. (1979) J. Am. Chem. Soc. 101:3832.
The crystal structure of highly fluorinated graphite fluorides, i.e., (CFx)n compounds with x>>0.5, has been investigated by several groups (Nakajima et al., Graphites, Fluorides and Carbon-Fluorine Compounds, CRC Press, Boca Raton, Fla., p. 84; Charlier et al. (1994) Mol. Cryst. Liq. Cryst. 244:135; Charlier et al. (1993), Phys. Rev. B 47:162; Mitkin et al. (2002) J. Struct. Chem. 43: 843; Zajac et al. (2000) J. Sol. State Chem. 150:286; Gupta et al. (2001) J. Fluorine Chem., 110-245; Ebert et al. (1974) J. Am. Chem. Soc. 96:7841; Pelikan et al. (2003) J. Solid State Chem. 174:233; and Bulusheva et al. (2002) Phys. Low-Dim. Struct. 718:1). The Watanabe group first proposed two phases: a first stage, (CF1)n, and a second stage, (CF0.5)n, the latter also commonly referred to as (C2F)n (Touhara et al., supra). In first stage materials, the fluorine is intercalated between each carbon layer to yield stacked CFCF layers, whereas in second stage materials, fluorine occupies every other layer with a stacking sequence of CCFCCF. Hexagonal symmetry was found to be preserved in both (CF1)n and (CF0.5)n phases. Theoretical crystal structure calculations were also carried out and different layer stacking sequences were compared using their total energy (Charlier et al. (1994), supra; Charlier et al. (1993) Phys. Rev. B 47:162; and Zajac et al., Pelikan et al., and Bulusheva et al., all supra).
(CFx)n compounds are generally non-stoichiometric with x varying between 0 and 1.3. For x<0.04, fluorine is mainly present on the surface of the carbon particles (Nakajima et al. (1999) Electrochemica Acta 44:2879). For 0.5≦x≦51, it has been suggested that the material consists of a mixture of two phases, (CF0.5)n and (CF1)n. “Overstoichiometric compounds,” wherein 1≦x≦˜1.3, consist of (CF1)n with additional perfluorinated —CF2 surface groups (Mitkin et al., supra). Surprisingly, although they have been reported in the literature (Kuriakos et al, supra; Nakajima et al. (1999) Electrochemica Acta 44:2879; and Wood et al. (1973) Abs. Am. Chem. Soc. 121), covalent type (CFx)n materials with x<0.5 have not been investigated in view of their crystal structure characterization. One possible reason of the focus on the fluorine-rich materials comes from their potential application as lubricants and as cathode materials for primary lithium batteries. In fact, for the latter application, the energy density of the battery, which is determined by its discharge time at a specific rate and voltage, has been found to be an increasing function of x.
The cell overall discharge reaction, first postulated by Wittingham (1975) Electrochem. Soc. 122:526, can be schematized by equation (1):(CFx)n+xnLinC+nxLiF  (1)
Thus, the theoretical specific discharge capacity Qth, expressed in mAh·g−1, is given by equation (2):
                                          Q            th                    ⁡                      (            x            )                          =                              x            ⁢                                                  ⁢            F                                3.6            ⁢                          (                              12                +                                  19                  ⁢                  x                                            )                                                          (        2        )            where F is the Faraday constant and 3.6 is a unit conversion constant.
The theoretical capacity of (CFx)n materials with different stoichiometry is therefore as follows: x=0.25, Qth=400 mAh·g−1; x=0.33, Qth=484 mAh·g−1; x=0.50, Qth=623 mAh·g−1; x=0.66, Qth=721 mAh·g−1; and x=1.00, Qth=865 mAh·g−1. It is interesting to note that even a low fluorine-containing (CF0.25)n material yields a higher theoretical specific capacity than MnO2, i.e., 400 mAh·g−1 versus 308 mAh·g−1, respectively. Despite the higher capacity, longer shelf life (on the order of 15 years), and substantial thermal stability of (CF0.25)n, MnO2 is the most widely used solid state cathode in primary lithium batteries, in part because of lower cost, and in part because of a higher rate capability.
The lower rate performance of Li/(CF) batteries is presumably due to the poor electrical conductivity of the (CF)n material. In fact, the fluorination of graphite at high temperature (typically 350° C.≦T≦650° C.) induces a dramatic change in the stereochemical arrangement of carbon atoms. The planar sp2 hybridization in the parent graphite transforms into a three-dimensional sp3 hybridization in (CFx)n. In the latter, the carbon hexagons are “puckered,” mostly in the chair conformation (Rudorff et al., Touhara et al., Watanabe et al., Kita et al., Charlier et al., Charlier et al., Zajac et al., Ebert et al., Bulusheva et al., and Lagow et al., all cited supra). Electron localization in the C—F bond leads to a huge drop of the electrical conductivity from ˜1.7 104 S·cm−1 in graphite to ˜10−14 S·cm−1 in (CF)n (Touhara et al., supra).
Accordingly, there is a need in the art for electrode materials that would compensate for the low conductivity of fluorinated carbon materials while preserving their high thermal stability and high discharge capacity. Ideally, such electrodes would enable, for example, the manufacture of lithium batteries having increased battery performance when discharged, particularly at high rates.